I would run a Focus Sash, since you’re going to use Sleep Powder/Quiver Dance before you GMAX anyway. Getting in a free Quiver Dance is pretty good, and then outspeeding the opponent and putting them to sleep with Sleep Powder is a good strategy. Butterfree isn’t bulky enough to pull off Quiver Dance without a Focus Sash, so I would go with that. The set would look like this, right?:
Butterfree-Gmax (M) @ Focus Sash
Ability: Compound Eyes
Shiny: Yes
EVs: 4 HP / 252 SpA / 252 Spe
Timid Nature
- Hurricane
- Quiver Dance
- Sleep Powder
- Pollen Puff
So, a Focus Sash is definitely the best option for Butterfree. You’ll be setting up before GMAXing anyway, and even though you probably won’t be OHKOed by anything while GMAXed, Quiver Dance and Sleep Powder should be used before that.
Hope this helps! :)